At least 51 migrants have died after being trapped inside a sweltering tractor-trailer truck found on Monday in Texas, 27 of whom were confirmed to be Mexican, Consul General of Mexico in San Antonio, Texas Ruben Minutti Zanatta said at a press conference on Tuesday.

Thirteen others, including three Mexicans, have been taken to hospital, he added.

Two suspects identified as Juan Francisco D'Luna-Bilbao and Juan Claudio D'Luna-Mendez, both Mexican citizens, have been charged with possessing firearms while residing in the U.S. illegally, according to court documents and U.S. authorities. 

Investigators traced the truck's vehicle registration to a San Antonio address that they placed under surveillance, and arrested the two men separately when each was seen leaving the residence.

A third suspect, described as a U.S. citizen who drove the truck, has also been taken into custody and was expected to be charged, but he remained hospitalized as of Tuesday evening, according to a Mexican official.

The migrants died inside a tractor-trailer in San Antonio, where temperatures swelled to a high of 103 degrees Fahrenheit (39.4 degrees Celsius). It was one of the worst disasters involving migrants in the United States in recent years, and comes five years after a similar deadly incident in the same central Texas city, a few hours' drive from the Mexican border.

The U.S. Immigration and Customs Enforcement (ICE) said that its Homeland Security Investigations division was conducting a criminal investigation into "an alleged human smuggling event" in coordination with local police.

The city's police chief, William McManus, on Monday said a person who works in a nearby building heard a cry for help and came out to investigate. The worker found the trailer doors partially opened and looked inside and found a number of dead bodies.

McManus said this was the largest incident of its kind in the city and said three people were in custody following the incident, although their involvement was not clear.

"The people that are responsible for subjecting other people to these conditions should be prosecuted to the fullest extent of the law," San Antonio Mayor Ron Nirenberg said during an interview on CNN on Tuesday.

The deaths once again highlight the challenge of controlling migrant crossings at the U.S.-Mexico border, which have reached record highs.

Lopez Obrador on Tuesday said he will meet with U.S. President Joe Biden in Washington on July 12 to discuss migration.

The I-35 highway near where the truck was found runs through San Antonio from the Mexican border and is a popular smuggling corridor because of the large volume of truck traffic, according to Jack Staton, a former senior official with ICE's investigative unit who retired in December.

In July 2017, 10 migrants died after being transported in a tractor-trailer that was discovered by San Antonio police in a Wal-Mart parking lot. The driver, James Matthew Bradley, Jr., was sentenced the following year to life in prison for his role in the smuggling operation.

(With input from Reuters, AFP)